著者

小野寺 龍太 祝 桂合 森田 泉

出版者

公益社団法人 日本金属学会

雑誌

日本金属学会誌 (ISSN:00214876)

巻号頁・発行日

vol.66, no.5, pp.521-527, 2002 (Released:2008-04-24)

参考文献数

24

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We examined the effect of grain size on flow stress and yield-point phenomena in variously heat-treated low-carbon steel specimens. We found that the experimental results are easily understood if the flow stress consists of two internal stresses, Yint and Yir. Yint is a common internal stress that is work hardened; Yir is the cause of yield-point phenomena, and its value is supposed to be rapidly decreased (work-softened) to zero with strain. Both Yint and Yir were affected by grain size. Furthermore, the effect of heat treatment was strong on Yint but weak on Yir. The Hall-Petch coefficient k was appreciably larger at lower yield points than in the large strain region. This comparison supports the hypothesis of the existence of two internal stresses because the grain size dependence of flow stress at lower yield points has two origins, Yint and Yir, whereas the latter case has only one origin, Yint. In the strain region exceeding yield-point elongation, the stress-strain curves of specimens with various grain sizes were perfectly superposed on each other by parallel translation, which means that if Yint is expressed as Yint(ε)=f(ε) for a specimen with a certain grain size, it is expressed as Yint(ε)=f(ε+Δε)−Δσ for arbitrary grain size specimens, where Δε and Δσ are the amounts of translation in strain and stress, respectively.

著者

小野寺 龍太 森川 龍哉 上木原 竜太

出版者

The Iron and Steel Institute of Japan

雑誌

鉄と鋼 (ISSN:00211575)

巻号頁・発行日

vol.87, no.1, pp.63-69, 2001-01-01 (Released:2009-06-19)

参考文献数

7

被引用文献数

1 1

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The yield point phenomena in low carbon steel have been explained by the Cottrell or the Johnston-Gilman theory in terms of microscopic variables such as dislocation density, dislocation velocity and the interaction energy between dislocation and solute atom. However, the lower yield stress and the amount of the Lüders strain (stretcher strain) seems not to have been predicted by the above theories. In the present work we tried to explain the phenomena quantitatively by a phenomenological (rheological) model using macroscopic variables such as work-hardening rate and the strain rate sensitivity of flow stress which we can be measured by tensile test. In the model a new internal stress was introduced which is supposed to be work (strain)-softened and age-hardened. Numerical calculation showed that the model can predict the values of the upper and lower yield stress and the amount of the Lüders strain, and the effect of strain rate on these characteristics.

著者

吾郷 真司 田中 照剛 横手 達夫 東田 賢二 小野寺 龍太

出版者

公益社団法人 日本金属学会

雑誌

日本金属学会誌 (ISSN:00214876)

巻号頁・発行日

vol.67, no.1, pp.34-39, 2003 (Released:2008-04-24)

参考文献数

1

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This study aims to give a mechanical explanation to manufacturing of metal (silver) foil in a traditional Japanese way in which the foils are put between Japanese papers and forged. It was clarified by forging tests carried out at various forging pressures that the forging pressure necessary for manufacturing silver foil is about 30 MPa, which is equal to one-tenth of the yield stress in a 90% rolled specimen. The test also showed that the plastic elongation of foil is of the order of 0.1% per forging (pressure is 30 MPa). This result suggests that the foil is not stretched by forging pressure, but by frictional tensile stress applied to the foil by the Japanese paper. Another test to measure the elastic property of paper showed that the Japanese paper was elongated by 0.5∼0.6% in the paper plane when it was compressed in the vertical direction at the pressure of 30 MPa. Considering the elastic strain of foil, the value of 0.5∼0.6% agreed well with the plastic elongation of foil of 0.1%. The forging tests also showed that the lower limit of foil thickness is dominated by the ability of paper not to adhere to the silver foil (removability); for example, it was 1 μm for some papers, but 0.4 μm for others.